Quiet Efficiency: Working Together to Reduce Subsea Noise

Thursday, April 18, 2013

With increased focus on the negative impact of subsea noise on the marine environment, regulators are considering action. Until then, different stakeholders are working together to develop a range of technical solutions to manage the issue. Over the past decade, regulators have sought to manage the impact the shipping industry has on the environment by introducing legislation covering ballast water treatment, restrictions on substances used in marine coatings and carbon emissions, among other issues. However, while subsea noise has long been recognised as a threat to marine life, there has been little action to manage the issue.
That may be changing. Last year, the IMO’s Sub-Committee on Ship Design and Equipment established a correspondence group on reducing commercial shipping noise through non-mandatory technical guidelines. The Committee identified four focus areas for noise reduction: propulsion, hull design, onboard machinery, and operational modifications. In Europe, the EU Marine Strategy Framework Directive is working with University researchers to assess noise pollution and define baseline levels of underwater noise. Once complete, the EU plans to introduce legislation setting standards to reduce noise levels by 2020. At the same time, the US-based National Oceanic and Atmospheric Administration (NOAA) is working on a global project to document human-generated subsea noise to create the world’s first sound maps.
In the meantime, most of the work to develop quieter ships so far has been driven by owners with commercial or scientific incentives to reduce subsea noise. According to DNV’s Kai Abrahamsen, Principle Engineer (Noise and Vibration), interest in this specialist field is growing. “Our work in this area has been confined to vessels active in seismic surveys, scientific research, commercial fishing and government-related naval projects. But we have also been engaged by energy companies, who seek to shield sensitive acoustic instrumentation related to sub-sea construction and communications from underwater noise,” he says. “We are encouraged by NOAA’s focus on subsea noise and statements made by the IMO and the EU suggesting that action may be taken to apply new standards for merchant vessels, which produce the most noise.”
Abrahamsen explains that low frequency sound waves can travel for hundreds of kilometres underwater before they dissipate, upsetting the highly evolved hearing abilities that fish and sea mammals (especially dolphins, whales and seals) rely on to hunt and communicate. “Reducing noise and vibration can be a complex engineering challenge, but we are seeing specialists in different areas working together to develop some highly innovative technical solutions,” he says. “A lot of subsea noise generated by ships can be traced to propellers, but some industry players are turning their attention to another significant contributor – engine vibration.”
Water is an efficient conductor of sound and engine and propulsion noise is amplified by a vessel’s hull. Underwater noise results from airborne noise, which is generated from the machinery to the structure of the vessel, and structure borne noise, which is generated from the vibrations of the machinery to the hull of the ship. To minimise this noise, engineers must consider a broad range of issues, including engine and generator mounting systems, base frames, materials and the position of isolators. One company working with noise and vibration is Pon Power Scandinavia, a leading distributor of Cat and MaK marine propulsion systems. In 2008, Pon Power began work on a mounting system designed to isolate engine vibration of high-speed gensets. According to Pon Power’s Area Service Manager, Øystein Skår, the development of the Genflex system was driven by the increasing power density of generator sets. “Over the last decade, engine output has been increasing. As a result, generators can no longer survive the mechanical noise and vibration produced by many of the new, more powerful diesel engines,” he says. “We identified a market for a mounting system that could reduce noise and vibration and be flexible enough to work with generators manufactured by different suppliers.”
Pon Power developed a system where the generator is rigidly mounted on easily adjustable chocks, and flexible mounts are fixed between the engine and base frame. Introduced in 2009, the Genflex concept has attracted significant industry interest. “The development process brought us into contact with a number of noise and vibration specialists around the world and has enabled us to carve out a strong reputation in a growing market,” he says. “This has lead to our participation in some interesting projects within noise and vibration.”
In 2008, Pon Power was asked by Siemens to provide generators for a diesel-electric powered research vessel under construction at the Jong Shyn Shipyard in Taiwan. The vessel, ORV Aerial, was ordered by Taiwan Ocean Research Institute to monitor sea life and perform acoustic surveys and high-resolution imaging of the seabed, among other tasks. To meet standards set by International Council for the Exploration of the Sea (ICES), the yard contacted DNV and the US-based company, Noise Control Engineering, to work with Siemens to ensure the vessel fulfilled the underwater noise requirements by reducing the vibrations and mobility levels of the gensets. Jeanette Jonasson, Project Manager for Pon Power Scandinavia, says the company assembled an international team to get the job done. “Our first challenge was to perform a lot of calculations to determine the optimal position and calibration of the mounts, isolators and base frame,” she says. “To get it right, we relied on other specialists to arrive at a good solution.”
Oriantek, a US-based company specializing in power train vibration and acoustic engineering, provided critical high-end dynamic analysis and layout schematics, including connections between engine and alternator and related pipe work. Jerry Vietinghoff, a 20-year veteran in the world of noise and vibration, says that the two companies have a long working relationship. “Oriantek supported Pon Power’s development of Genflex and has worked with Caterpillar on a number of noise and vibration projects over the years,” he says. “It is a real advantage to work with familiar faces who know what they are doing.”
Jonasson says Oriantek’s work led to a unique solution. “We selected highly flexible and resilient engine mounts and seismic masses in a three-stage isolation scheme and added a flexible coupling between the engine and the generator,” she says. “By placing isolators between the base frame and the blocks, and the blocks and the foundation, our calculations indicated that we could reduce vibration levels significantly, especially between the rotor and stator.” The design also included the addition of an extra mass of 15 tons below the flexible skid to further reduce vibration to the hull.
Pon Power turned to Christie & Grey, a leading manufacturer of precision anti-vibration and isolation mounts. Patrick Bergin, lead engineer on the project, says the company applied its proprietary design software to calculate the natural frequencies and response to seaway motion of five different masses (the diesel engine, the alternator/base frame and three cast iron blocks) to properly design the resilient mounting system. “We had worked on three stage systems in the past, but never one that accounted for so many masses and frequencies,” he says. “I’ve worked in this field for many years and believe this system is not only unique, but represents a big step forward in structural noise control.” The calculations were then completed and verified by the shipyard.
After more than a year, the team assembled at Pon Power Scandinavia‘s facility in Esbjerg, Denmark for the Factory Acceptance Test (FAT). Other participants included a large delegation from Taiwan, including government officials and shipyard personnel, and representatives from Siemens, Oriantek, Christie & Grey and Noise Control Engineering. DNV was also present to verify that the solution complied with ICES. “The test won approval and, save for a few minor adjustments, the project was a complete success,” says Jonasson. “Members of our team were on hand during installation and were very impressed by the care the yard had taken on the foundation,” she says. “This was one critical part of the project we did not control, but they did a fantastic job.”
Launched in 2012, the Ocean Research Vessel (ORV) Aerial completed sea trails and is now in active service. DNV’s Abrahamsen, who followed the development process from beginning to end, says that demand for innovative technical solutions to manage noise and vibration will continue to rise. “Increasingly, noise and vibration have been included in environmental impact studies for coastal projects. Also energy companies with operations at sea have started to apply stricter requirements on underwater sound for offshore support vessels,” he says.
“This project is a good example of how different industry stakeholders can work together to develop innovative solutions that, pending more action from regulators, will have a positive, long-term impact on fragile marine eco-systems all over the world.”